Borosilicate glass having improved UV transmission, thermal and chemical properities and methods of making and using same

ABSTRACT

The borosilicate glass has a UV transmission of at least 80% at a UV wavelength of about 254 nm for a borosilicate glass sheet thickness of 2 mm, a linear coefficient of thermal expansion, α 20/300 , of 3.2 to 3.4×10 -6  K -1 , a hydrolytic resistance of class 1 and a composition in % by weight based on oxide of 
     
         ______________________________________                                    
 
    
     glass formers      &lt;96%,                                                  
SiO 2           from 79.0 to 81.0%,                                    
B 2  O 3     from 12.5 to 13.0%,                                    
Al 2  O 3    from 2.0 to 4.0%,                                      
alkali metal oxides                                                       
                   &gt;2%,                                                   
______________________________________                                    
 
     wherein said alkali metal oxides include from 2.0 to 3.5% said K 2  O and from 1.0 to 2.0% said Li 2  O,______________________________________alkaline earth metal &lt;0.3%,oxides + ZnOreducing agent       0.025 to 2%, andnon-oxidizing fining agent                     0 to 3%.______________________________________ 
     The weight ratio of K 2  O to Li 2  O is from 2:1 to 1:1 in the borosilicate glass. The glass is particularly suitable as a UV transparent material for EPROM windows, UV lamps, photomultipliers, spectrophotometers, particularly any of the foregoing used under particularly difficult climatic conditions, and as a protective tube for UV lamps immersed in water in waste water treatment plants, in UV oxidation reactors and/or solar reactors.

BACKGROUND OF THE INVENTION

The present invention relates to an improved borosilicate glass and,more particularly, borosilicate glass having a high transmission forelectromagnetic radiation in the UV region, a small thermal expansioncoefficient and a high chemical stability. It also relates to a methodof making the improved borosilicate glass and a method of using it.

Glasses having high UV transparency have numerous applications. They areused, for example, as EPROM windows and UV glasses, as windows inphotomultipliers and spectrophotometers, and as protective tubes of UVlamps in UV reactors.

UV oxidation reactors are used to decrease or eliminate pollutants fromchemically and/or biologically contaminated water in the most variedmanner or for sterilizing drinking water.

The oxidative degradation of chemical compounds such as chlorinatedhydrocarbons, CFCs, AOX or BTX and the oxidative destruction of bacteriain water is promoted by UV radiation--principally at a wavelength 253.7nm. A UV radiation protection tube, which can be universally used in allUV oxidation reactor types, must therefore, in addition to high UVtransmission at 254 nm, have very good chemical stability since it isexposed to attack by aqueous solutions over a very long period andpossibly at elevated temperature. Since the UV oxidation reactors canalso be exposed to high temperature stresses in operation, the radiationprotection tube must have as high as possible a resistance totemperature change which can be achieved by low thermal expansion.

Quartz glass is highly suitable for use per se as a UV-transparentglass, but because of its high price and processing difficulties it isonly used in exceptional cases requiring particularly good hydrolyticproperties. A further disadvantage with quartz glass is its poorfusibility to ceramic substrates (e.g. Al₂ O₃), Ni--Fe--Co-alloys ormolybdenum, because of a thermal expansion coefficient which is toosmall.

"Borosilicate glass 3.3", a borosilicate glass having minimum thermalexpansion--with the highest possible chemical stability--and thus highresistance to temperature change is currently known and marketed.

Because of its small thermal expansion coefficient, α_(20/300), of3.3×10⁻⁶ K⁻¹ and its high chemical stability, this glass has assumed anoutstanding position internationally, e.g. in the areas of laboratoryequipment, chemical apparatus, piping, fittings and household glass.

It is traded under various brand names--such as Duran, Pyrex, Rasothermand Simax and has a very similar chemical composition which isapproximately (% by weight) SiO₂, 80 to 81%; B₂ O₃, 12 to 13%; Al₂ O₃, 2to 3% and Na₂ O+K₂ O, 4 to 5%.

For Duran®, according to Laboratory Glass Catalog No. 50020/1991, thecomposition (in % by weight) below is given by the manufacturer as:SiO₂, 81%; B₂ O₃, 13%; Al₂ O₃, 2%; and Na₂ O+K₂ O, 4%.

However, "Borosilicate glass 3.3", because of its composition, has aninsufficient UV transparency in the UV-B range of 280 to 315 nm. Forwavelengths below approximately 300 nm, it is insufficientlytransparent, even if the glass is produced with raw materials extremelylow in Fe₂ O₃.

In the past, it has apparently not been recognized in developmental workinvolving "Borosilicate glass 3.3" that it is possible to influence itsstructure-related insufficient UV transparency decisively by a specificstructural change.

Thus, German Patent DE-C 767,476 long ago disclosed a borosilicate glasshaving the following composition: (in % by weight): from 72.5 to 83%SiO₂ +Al₂ O₃ ; from 0 to 15% B₂ O₃ ; from 4 to 16% alkali metal oxidesand alkaline earth metal oxides.

From U.S. Pat. No. 3,258,352, those skilled in the art know of a glasshaving the following composition (in mol. -%)>75% SiO₂ +B₂ O₃ +Al₂ O₃ ;from 0 to 20% Al₂ O₃ ; from 0 to 50% B₂ O₃ ; from 0 to 20% Li₂ O+Na₂O+K₂ O; from 0 to 30% MgO+CaO+SrO+BaO; and from 0 to 20% PbO.

JP-B 92/33741 B2 teaches a glass having the following composition (in %by weight) from 68 to 82% SiO₂ ; from 0.5 to 5% Al₂ O₃ ; from 10 to 18%B₂ O₃ ; from 3.5 to 8% Li₂ O+Na₂ O+K₂ O; from 0 to 3% CaO+MgO; and from0.05 to 1% fining agent.

It cannot be inferred from the prior art that there is a borosilicateglass of the type "Borosilicate glass 3.3" having a high transmission inthe UV region.

The only highly UV-transparent material with low thermal expansion andhigh chemical stability currently known is quartz glass or silica glass.

However, this single-component glass, as already mentioned at theoutset, is very expensive and can be produced only with difficulty ornot at all in complicated geometric dimensions and narrow tolerances.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a borosilicate glassof the type "Borosilicate glass 3.3", having similar properties ofthermal expansion, viscosity, chemical stability, meltability andcrystallization resistance, but which also has a high UV transmission ofat least 80% at a wavelength of 254 nm for a 2 mm sheet thickness, whichhas a good to very good hydrolytic resistance of class 1 according toDIN 12 111, which has a linear thermal expansion coefficient,α_(20/300), of 3.2 to 3.4×10⁻⁶ K⁻¹ and which can be produced stably,especially with respect to the good UV transparency, i.e. that slightchanges in the reduction conditions for the iron (III) ion have noinfluence, or only an extremely small influence, on the UV transmission.

According to the invention this object is achieved by a borosilicateglass having a composition (in % by weight based on oxide)

    ______________________________________                                        glass formers           <96                                                   SiO.sub.2 + B.sub.2 O.sub.3 + Al.sub.2 O.sub.3                                                        <98                                                   alkali metal oxides     >2                                                    K.sub.2 O:Li.sub.2 O    2:1-1:1                                               alkaline earth metal oxides + ZnO                                                                     <0.3                                                  reducing agent          0.025 to 2                                            non-oxidizing fining agent                                                                            0 to 3.                                               ______________________________________                                    

Surprisingly, it has been found that the known "borosilicate glass 3.3",by modifying its alkali metal oxide content, can be structurally changedso that a highly UV-transparent matrix results which makes possible UVtransparencies of τ₂₅₀ nm/2 mm >80% and at the same time the specialcombination of properties of low thermal expansion and high chemicalstability can be retained.

In borosilicate glasses, the UV transparency of the glass for a givenimpurity content of UV absorbers is determined by the ratio of BO₃ toBO₄ coordination groups.

High UV transparency requires a structure in which as many boron atomsas possible are present as BO₃ structural elements in Boroxol rings (B₃O₆).

Since some Raman spectroscopy studies established that only such astructure guarantees high UV transparency, it became apparent thatattempts to improve the structure of "Borosilicate glass 3.3" should bemade considering these findings.

Surprisingly, this type of structural optimization was possible byintroducing defined amounts of K₂ O and Li₂ O at approximately unchangedcontents of SiO₂, B₂ O₃ and Al₂ O₃. In laboratory tests, it wasdetermined that the use of 2.0 to 3.5% by weight of K₂ O and 1.0 to 2.0%by weight of Li₂ O, keeping the weight ratio of K₂ O to Li₂ O from about2:1 to 1:1, results in a glass which, in its essential physicochemicalproperties--including the improved UV transparency--approaches"Borosilicate glass 3.3".

The thermal expansion coefficient, α_(20/300), should be from 3.2 to3.4×10⁻⁶ K⁻¹ and the chemical stability, with the hydrolytic class 1 oracid class 1, likewise corresponds to "Borosilicate glass 3.3".

With respect to the melting and fining behavior, the viscosity at hightemperatures, the crystallization stability and the processability, theglass according to the invention is likewise very similar to thecomparison glass Duran®.

The borosilicate glass according to the invention therefore has acomposition (in % by weight based on oxide) of from 79.0 to 81.0% SiO₂ ;from 12.5 to 13.0% B₂ O₃ ; from 2.0 to 4.0% Al₂ O₃ ; from 2.0 to 3.5% K₂O; from 1.0 to 2.0% Li₂ O; from 0.025 to 2.0% reducing agent and from 0to 3.0% non-oxidizing fining agent.

Studies have shown that it is necessary, in order to achieve very highUV transmission rates of

τ₂₅₀ nm/2 mm >80%, to use raw materials so low in iron that no more thanapproximately 5 ppm of Fe₂ O₃ are contained in the glass.

All agents reducing Fe³⁺ to Fe²⁺ can be used as reducing agent.

In the laboratory melts in an air atmosphere in an electrically heatedfurnace, glasses according to the invention having the best UVtransparencies resulted when metallic silicon was used as reducingagent. On the order of about 300 ppm of metallic silicon was used.

Each melting process requires a type and amount of reducing agentmatched to the specific conditions.

Surprisingly, the redox potential necessary for the reduction of Fe³⁺,because of the basicity of the glasses according to the invention, canrelatively easily be kept constant. The high UV transparency, τ₂₅₄ nm/1mm, of up to 87/88%, which is sufficient for EPROM glasses, isrelatively insensitive to deviations in melt technology.

On the one hand, an excess of reducing agents does not cause undesirableabsorptions and, on the other hand, the reduced Fe²⁺, despite access toair, can be kept at the desired valency.

This stability of high UV transparency represents a great advantage incomparison to other glasses.

The fining of the glass was carried out with chlorides, preferably withpotassium chloride (KCl and in part with small amounts of sodiumchloride, NaCl).

In the selection of fining agents, it must be noted that onlynon-oxidizing agents are used which, in addition, must also contain noUV absorbers, so that, e,g., no nitrates, sulfates, As₂ O₃, Sb₂ O₃ andCeO₂ can be used.

In Table 1, compositions of glasses according to the invention and theraw materials used for melting are listed.

The raw materials were used as quartz flour (trade name Sipur orYotaquartz), boric acid, aluminium hydroxide, potassium carbonate,lithium carbonate and chlorides in very pure optical qualities havinglow Fe₂ O₃ contents.

For melting, crucibles having a volume of approximately 1 liter andquartz glass stirrers were used. The melt temperatures in theelectrically heated laboratory furnace were 1630°-1650° C., the meltingtime approximately 6-7 hours.

The melts were carried out in conventional manner in an air atmosphere,poured into molds and cooled in a stress-free manner.

In Table 2, important physicochemical properties of a glass according tothe invention are compared with "Borosilicate glass 3.3" as exemplifiedby Duran® 8330.

It is clear that the glass according to the invention has identical orhighly similar properties to Duran®, including the increased UVtransparency.

In the comparison of UV transmission properties, glasses having similarlevels of impurities absorbing in the UV range must be compared. Forthis reason, Duran® was produced on a laboratory scale using optical,i.e. very pure, low-iron raw materials, under the same conditions as theglass according to the invention, so that the precipitously increased UVtransparency can actually be related to structural optimization of theglass.

The comparison in Table 2, moreover, shows that the glass according tothe invention has lower viscosities so that it can be produced withlower energy and in better quality.

                  TABLE 1                                                         ______________________________________                                        Composition of glasses according to the invention                                     Glass 1      Glass 2    Raw                                           Oxide   % by weight  % by weight                                                                              materials                                     ______________________________________                                        SiO.sub.2                                                                             79.50        80.50      Sipur/Yota                                    B.sub.2 O.sub.3                                                                       12.70        12.70      H.sub.3 BO.sub.3                              Al.sub.2 O.sub.3                                                                      3.40         2.40       Al(OH).sub.3                                  K.sub.2 O                                                                             1.40         2.0        K.sub.2 CO.sub.3                              K.sub.2 O                                                                             1.50         --         KCl                                           Li.sub.2 O                                                                            1.50         1.50       Li.sub.2 CO.sub.3                             Na.sub.2 O                                                                            --           0.9        NaCl                                          Si°                                                                            0.03         0.03       Si°, metallic                                  100.03       100.03                                                   ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Physicochemical properties of glass according to the                          invention in comparison with Duran® 8330                                                  Example 2                                                                             Duran 8330                                            ______________________________________                                        α.sub.20/300 (10.sup.-6 K.sup.-1)                                                         3.3       3.25                                              ρ (g/cm.sup.3)                                                                              2.2       2.23                                              T.sub.g (°C.)                                                                            520       530                                               E.sub.W (°C.)                                                                            785       815                                               V.sub.A (°C.)                                                                            1250      1270                                              Hydrolytic class  1         1                                                 DIN 12111, ISO 719                                                            Acid class        1         1                                                 DIN 12116, ISO 1776                                                           Alkali class      3         2                                                 DIN 52322, ISO 675                                                            Crystallization range (°C.)                                                              680-1040  680-1030                                          CR.sub.max (μm/h)                                                                            40        40                                                Transmission rates in %                                                       (highly pure materials)                                                       τ.sub.250 nm 2 mm                                                                           86        30                                                τ.sub.250 nm 10 mm                                                                          63        10                                                ______________________________________                                    

While the invention has been illustrated and embodied in a borosilicateglass having a high transmission for electromagnetic radiation in the UVregion, a small thermal expansion coefficient and a high chemicalstability and a method of making and using it, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and desired to be protected by Letters Patent isset forth in the appended claims.
 1. Borosilicate glass having a high UVtransmission, a small linear coefficient of thermal expansion, a highchemical stability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers      <96%,                                                      SiO.sub.2          from 79.0 to 81.0%,                                        B.sub.2 O.sub.3    from 12.5 to 13.0%,                                        Al.sub.2 O.sub.3   from 2.0 to 4.0%,                                          alkali metal oxides                                                                              >2%,                                                       ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal <0.3%,                                                   oxides + ZnO                                                                  reducing agent       0.025 to 2%, and                                         non-oxidizing fining agent                                                                         0 to 3%;                                                 ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1. 2.Borosilicate glass as defined in claim 1, further comprising up to 5 ppmof iron oxides.
 3. Borosilicate glass as defined in claim 1, whereinsaid UV transmission is at least 80% at a UV wavelength of about 254 nmfor a borosilicate glass sheet thickness of 2 mm and said linearcoefficient of thermal expansion, α_(20/300), is from 3.2 to 3.4×10⁻⁶K⁻¹ ; and having a hydrolytic resistance of class 1 according to DIN
 12111. 4. Process for making a borosilicate glass having a composition in% by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3%,                                                  reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a ratio K₂ O:Li₂ O is 2:1-1:1, said process comprising thesteps of making a glass melt in air atmosphere in an electrically heatedfurnace and adding metallic silicon as at least a part of said reducingagent to said glass melt.
 5. Process as defined in claim 4, wherein saidcomposition of said borosilicate glass made by the process includes upto 5 ppm of iron oxides, and wherein said metallic silicon is added tosaid glass melt until about 300 ppm of said silicon is present in saidglass melt as a reducing means for any Fe⁺³ present in said iron oxides.6. Process for making a borosilicate glass having a composition in % byweight based on oxide of from 79.0 to 81.0% SiO₂ ; from 12.5 to 13.0% B₂O₃ ; from 2.0 to 4.0% Al₂ O₃ ; from 2.0 to 3.5% K₂ O; from 1.0 to 2.0%Li₂ O; from 0.025 to 2.0% reducing agent and from 0 to 3.0%non-oxidizing fining agent, said process comprising the steps of makinga glass melt in air atmosphere in an electrically heated furnace andadding metallic silicon as an Fe⁺³ -reducing agent to said glass meltuntil said melt is about 300 ppm silicon.
 7. In a UV lamp comprising aUV-transparent glass, the improvement wherein said glass is aborosilicate glass having a high UV transmission, a small linearcoefficient of thermal expansion, a high chemical stability, and acomposition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3 %,                                                 reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.
 8. In aprotective tube comprising a UV-transparent glass, the improvementwherein said glass is a borosilicate glass having a high UVtransmission, a small linear coefficient of thermal expansion, a highchemical stability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3 %,                                                 reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.
 9. In anEPROM window comprising a UV-transparent glass, the improvement whereinsaid glass is a borosilicate glass having a high UV transmission, asmall linear coefficient of thermal expansion, a high chemicalstability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3 %,                                                 reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.
 10. In aphotomultiplier comprising a UV-transparent glass, the improvementwherein said glass is a borosilicate glass having a high UVtransmission, a small linear coefficient of thermal expansion, a highchemical stability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3 %,                                                 reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.
 11. In aspectrophotometer comprising a UV-transparent glass, the improvementwherein said glass is a borosilicate glass having a high UVtransmission, a small linear coefficient of thermal expansion, a highchemical stability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3 %,                                                 reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.
 12. In asolar reactor comprising a UV-transparent glass, the improvement whereinsaid glass is a borosilicate glass having a high UV transmission, asmall linear coefficient of thermal expansion, a high chemicalstability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3%,                                                  reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.
 13. Inan apparatus containing a UV-transparent glass, the improvement whereinsaid UV-transparent glass is a borosilicate glass having a high UVtransmission, a small linear coefficient of thermal expansion, a highchemical stability, and a composition in % by weight based on oxide of

    ______________________________________                                        glass formers   <96%,                                                         SiO.sub.2       from 79.0 to 81.0%,                                           B.sub.2 O.sub.3 from 12.5 to 13.0%,                                           Al.sub.2 O.sub.3                                                                              from 2.0 to 4.0%,                                             alkali metal oxides                                                                           >2%,                                                          ______________________________________                                    

wherein said alkali metal oxides include from 2.0 to 3.5% K₂ O and from1.0 to 2.0% Li₂ O,

    ______________________________________                                        alkaline earth metal oxides + ZnO                                                                   <0.3%,                                                  reducing agent        0.025 to 2%, and                                        non-oxidizing fining agent                                                                          0 to 3%;                                                ______________________________________                                    

and wherein a weight ratio of K₂ O to Li₂ O is from 2:1 to 1:1.